1. Field of the Invention
The present invention concerns a method for monitoring a radio frequency power amplifier, as well as a corresponding radio-frequency device, a corresponding radio-frequency monitoring device and a corresponding magnetic resonance tomography system.
2. Description of the Prior Art
Magnetic resonance tomography is an imaging modality now in widespread use that is based on the detection of signals arising from precessing nuclear spins of protons in a body region of an examination subject. First, a strong, stable homogenous magnetic field is generated in which the body region is disposed, which causes a stable alignment of the protons in the body region. This stable alignment is altered by radiating electromagnetic radio frequency energy into the region. After this excitation the magnetic resonance signals created in the body are detected with suitable receiver coils. The signals are processed are processed and an image of the tissue in this body region is reconstructed therefrom, with which a medical diagnosis can be made, or a surgical procedure can be planned or guided.
A magnetic resonance tomography system has a number of interacting components, each of which requires the use of modern and elaborate technologies. A central component of a magnetic resonance tomography system is the radio frequency device. This is responsible for the generation of the radio frequency pulses that are radiated into the body region to be imaged. The radio frequency pulses at the output of a radio frequency power amplifier of a magnetic resonance tomography system are conducted via a measurement device to a transmission coil that radiates the radio frequency pulses into the body region. As used herein “transmission coil”, means any antenna device with which the radio frequency pulses can be radiated.
With the development and establishment of magnetic resonance tomography systems, limit values to ensure patient safety have been standardized that regulate the maximum permissible radio-frequency irradiation into a human body. A typical limit value for this is the maximum allowable SAR value (SAR=specific absorption rate).
To abide by these limit values, in the measurement device cited above, measurement values are recorded that represent the power of the radio frequency pulses radiated by the transmission coil. Control values are formed on the basis of a number of such measurement values. These control values are then compared with a fixed threshold (limit control value) that is predetermined by a standard, and the radio frequency power amplifier is automatically limited in operation (usually deactivated) if and when a control value exceeds the predetermined threshold.
To take into consideration system-dependent measurement errors, it is known to subtract a system error from the predetermined threshold and to compare the effective threshold thus obtained with the control values to monitor the radio-frequency power amplifier.
Investigations have shown that each individual measurement is afflicted with at least the following three measurement errors of different types:                errors via directivity of the directional coupler,        calibration errors and        linearity errors.        
The directivity error depends on the degree of the reflection and is dependent on the load of the transmission coil, i.e. it is dependent on how much mass is presently located in the transmission coil. The degree of the reflection is determined by an adjustment measurement before a measurement period. Both of the last types of errors are dependent on the magnitude of the measurement value, and the error curve is non-proportional, such that given small measurement values the relative measurement error is larger than given large measurement values.
Conventional methods account (as described above) for the cited errors by subtracting an all-inclusive system error from the threshold in a later processing stage, independent of the individual measurement values. Given the determination of this system error, the measurement errors of the individual measurements and the measurement values of the individual measurements are not known. Therefore, for the existence of safety, at least some larger measurement errors has to be assumed in the determination of the system errors. This leads to the assumption of a larger system error than actually exists. The assumption of an unnecessarily small effective threshold results from this, which can lead to an unnecessarily premature deactivation of the radio-frequency power amplifier as a result.
The system errors introduced into the effective threshold are thus taken as a whole, such that a deactivation of the radio-frequency power amplifier frequently also ensues when the actual radio-frequency irradiation in a body region is still far removed from the legal threshold. However, a lower radio-frequency power in the measurement leads directly to a reduced dynamic range and consequently leads to a quality loss in the generated exposure.